Systems have been described wherein pen- or stylus-like devices sense their own movement relative to a surface, for the purposes of providing input to a computer system, by detecting information which is invisibly but machine-readably encoded on the surface. See for example U.S. Pat. No. 5,477,012, U.S. Pat. No. 5,652,412 and PCT application WO99/50787. Visible information intended for human consumption may also be placed on the surface, although typically in a separate step which therefore may include recording an association between the two kinds of information. The encoded information doesn't include knowledge of the content of the visible information, and any content-specific feedback provided to a user during the user's interaction with the encoded information must be provided on the basis of the recorded association. If the recorded association is remote or unavailable, timely feedback may be impossible to provide.